1. Field of the Invention
The present invention relates to the technical field of channel estimation and, more particularly, to a Gigabit Ethernet channel estimation device and its estimation method.
2. Description of Related Art
For conventional communication system, Inter-symbol-interference (ISI) occurs if received signals on a receiving end are commonly affected due to the limited bandwidth of transmission channels. Accordingly, a Decision Feedback Equalizer (DFE) is used to eliminate ISI. The DFE includes a feedforward equalizer (FFE) and a feedback equalizer (FBE). However, for a receiver system of a Gigabit Ethernet as shown in FIG. 1, in addition to the channel impairment, the convergence of DFE, timing recovery, echo cancellation, and NEXT (Near End Cross Talk) loop cancellation are also required to be accomplished when receiving the signals. Thus, circuitry for DFE convergence, timing recovery, echo cancellation, and NEXT loop cancellation are set in the receiver system. In order to avoid system divergence caused by interaction of those circuitry, the conventional approach is to directly set the optimized coefficients for those circuitry in initialization for convergence. The optimized coefficients are determined through channel estimation.
CAT-5 cable is defined as a transmission medium in IEEE 802.3u and 802.3ab standards. The CAT-5 cable is a time-invariant channel, which can be tested for obtaining the coefficients based on different transmission line-length in advance. For an MLT-3 transmission format applied to IEEE 802.3u (Fast Ethernet), the transmission line-length can be determined according to a received signal on a receiving end and accordingly applied to a mapping or conversion table to determine the coefficients. However, for a PAM-5 signal format used in a Gigabit Ethernet network, the transmission line-length cannot be estimated using the aforementioned methods. Additionally, the aforementioned methods have poor estimation accuracy in some circumstances (such as when the power of the transmitter is too high or too low). Another method of line-length estimation is to measure received signal energy. However, this method has the same disadvantages as compared with the others.
Since the CAT-5 channel is a kind of time-invariant environment, coefficients of a corresponding DFE or A/D auto-gain controller (AGC) can be determined if line-length can be accurately estimated.
The conventional method to estimate transmission line-length is through determining the relationship between the frequency spectrum of received signal and the transmission line-length. However, the low frequency response of received signal is greatly affected by transformer. Thus the variance of low frequency response of received signal is large with respect to different transformers manufactured by different manufacturers. Further, since most noise is present in the high frequency part, the variance of high frequency spectrum response of received signal is also large. Therefore, both the low and high frequency parts of received signal are not suitable for line-length estimation. According to simulation, a ratio of frequency response of 6 and 43 MHz can be used for transmission line-length estimation. However, in practice, the cost for hardware implementation is very high due to a lot of multiplication and addition computations are required for executing discrete Fourier transform (DFT).
Therefore, it is desirable to provide an improved channel estimation device and method to mitigate and/or obviate the aforementioned problems.